The present invention generally relates to an image sensor for converting an incident electromagnetic wave such as a light beam or an X-ray into electric charge and outputting an image signal by sequentially reading out the electric charge, and also relates to a method of manufacturing such image sensor.
A known active matrix substrate for use in a liquid crystal display device, etc., includes a plurality of independently driven pixel electrodes arranged in a matrix form, and switching elements such as TFTs (Thin Film Transistors), etc., provided for respective pixel electrodes. In the liquid crystal display device adopting such active matrix substrate, an image is displayed by sequentially selecting the switching elements by scanning lines and reading potentials of signal lines into the pixel electrodes via the switching elements.
The foregoing active matrix substrate can be used for an image sensor. Examples of known image sensors adopting the active matrix substrate include: an image sensor including a conversion layer formed on an upper layer of the active matrix substrate, for directly converting incident electromagnetic wave such as a light beam, an X-ray, etc., into electric charge, wherein the electric charge generated from the conversion layer is stored in pixel capacitance at high voltage, and the electric charge is read out sequentially from the pixel capacitance. For example, Japanese Unexamined Patent Publication No. 212458/1992 (Tokukaihei 4-212458) published on Aug. 4, 1992, discloses an image sensor of the above type wherein electric charge as generated by the conversion layer is stored in auxiliary capacitance, and data (potential data) are stored in respective pixels in the form of electric charge according to the characteristics of an object. As in the case of the aforementioned liquid crystal display device, by sequentially scanning the scanning lines, for example, the data stored in a pixel selected by a scanning line is read out and transmitted via a switching element to a signal line, and an image projected to the image sensor is read out from a circuit such as an operation amplifier provided on the other end of the signal line.
The active matrix substrate, which is a precursor to the sensor in the foregoing example can be manufactured at low costs without requiring any additional facilities, because the manufacturing process for liquid crystal display devices can be used for the manufacturing process of image sensors only by adjusting the dimensions of the pixel capacitance and the time constants of the switching elements to be optimal for image sensors.
FIG. 6 is a cross-sectional view illustrating a schematic structure of a known example of the basic image sensor adopting an active matrix substrate. The structure illustrated in FIG. 6 is disclosed in AM-LCD""99 xe2x80x9cReal-time Imaging Flat Panel X-Ray Detectorxe2x80x9d by M. Ikeda, et al. As illustrated in FIG. 6, the active matrix substrate of this sensor is prepared by forming a switching element 51 on a transparent insulating substrate 55, and further vapor-depositing thereon a conversion layer 66 and a metal layer 67 in this order. The switching element 51 is prepared by forming on the transparent insulating substrate 55, a gate electrode 56, an auxiliary capacitance electrode (not shown), a gate insulating film 57, a semiconductor layer 58, an n+-Si layer 59 to be patterned into a drain electrode, a metal layer 60 and a transparent electrically conductive film 61 to be patterned into a source signal line, and a protective film 62 in this order, thereby forming a substrate of the image sensor. The conversion layer 66 is provided for converting an X-ray into electric charge. The metal layer 67 is patterned into an electrode for use in applying a voltage to the conversion layer 66. In the foregoing structure, the transparent electrically conductive film 61 is patterned into the pixel electrodes for storing the electric charge as converted in the conversion layer 66.
In the image sensor, the electric charge is read out from respective pixel electrodes in contrast to the liquid crystal display device in which electric charge is applied to the pixel electrodes. Therefore, if a normal readout operation of a predetermined cycle is not performed due to any failure, or a trouble in signal readout program, unexpectedly large electric charge may be stored in the pixel electrode, and the resulting high voltage may cause a damage on the active matrix substrate. The foregoing problem is discussed in xe2x80x9cCharacteristics of dual-gate thin film transistors for applications in digital radiologyxe2x80x9d (NRC""96) in xe2x80x9cCan. I. Phys. (Suppl) 74 published in 1996, in which the following structure has been proposed as a solution to the problem. That is, a pixel electrode is extended over a switching element, so that the pixel electrode can be functioned as one of the gate electrodes of a dual-gate transistor, and at or above a predetermined threshold voltage, the transistor is switched on, and excessive electric charge is released.
The structure of an image sensor which is particularly effective in preventing the foregoing problem will be explained in reference to FIG. 7. As illustrated in FIG. 7, the image sensor has a so-called xe2x80x9cmushroom structurexe2x80x9d wherein pixel electrodes 72 and source lines 71 are formed in different layers so as to be insulated by an insulating layer 73 formed in-between, so that the entire channel region W of a transistor 74 is covered with the corresponding pixel electrode 72. In FIG. 7, the reference numerals 75, 76, 77, 78 and 79 indicate a gate electrode, a drain electrode, an auxiliary capacitance, a conversion layer and a semiconductor layer respectively.
The foregoing structure of Waechter, et al, illustrated in FIG. 7 is effective for the high voltage protection in the pixel electrodes 72. As to the size of the pixel electrodes 72, however, significant improvement from the aforementioned active matrix substrate illustrated in FIG. 6 cannot be expected. It is generally known that the larger is the area occupied by the pixel electrodes 72, the more efficiently, the electric charge generated from the conversion layer 78 can be collected in the pixel electrodes 72. In the generally used active matrix substrate, however, there is a limit for an increase in size of each pixel electrode as pixel electrodes are arranged in a plane with certain intervals from source bus lines.
In the foregoing structure of FIG. 7 wherein the insulating film 73 is formed between the source line 71 and the pixel electrodes 72, the pixel electrodes 72 can be formed over the source lines 71 while maintaining the insulation between them. In this state, the electrostatic capacitance is generated between the pixel electrodes 72 and the source lines 71, and an overall capacitance of the source lines 71 when seen from the side of the signal readout circuit increases, and a noise of the readout signal is increased, resulting in lower signal to noise (S/N) ratio. For the foregoing reasons, the structure of FIG. 7 would not offer any significant improvement in size of the pixel electrodes 72 from the conventional active matrix substrate.
In the X-ray image sensor, generally a large pixel capacitance is ensured. For this reason, the capacitance between the pixel electrode 72 and the source line 71 becomes a load capacitance to the source line 71 directly. On the other hand, internal noise generated in the signal readout amplifier is amplified by a gain in proportion to the ratio of the capacitance of the source line 71 to the feedback capacitance. It is therefore effective to reduce the capacitance of the source line 71 for a reduction in internal noise.
Further, an increase in capacitance of the source line 71 may cause variations in potential of the source line 71 corresponding to the capacitance CsD (per pixel) between the pixel electrode 72 and the source line 71 with changes in pixel potential corresponding to the part of the image irradiated with an X-ray. For example, when reading out signals via the source line 71 with a selection of certain scanning line, the electric charge is kept being stored in other pixel electrodes, while the electric charge in positive polarity and in proportion to the capacitance CsD are being stored in the source line 71. The amount of the electric charge to be stored in the pixel electrodes and the source line 71 differ depending on the image on an entire screen, thereby presenting a problem that a so-called crosstalk is generated when reading out signals as being affected by pixel electrodes aligned in direction parallel to the source line 71.
In order to reduce the capacitance of the source line 71, for example, an image sensor adopting an interlayer insulating film made of photosensitive resin has been proposed, for example, in xe2x80x9cSimilarities between TFT Arrays for Direct-Conversion X-Ray Sensors and High-Aperture AMLCDsxe2x80x9d (SID 98 DIGEST) by W. den Boer, et al, published in 1998.
However, W. den Boer, et al does not refer to the dual-gate structure.
It is an object of the present invention to provide an image sensor of a dual-gate structure which permits excessive electric charge to be released effectively while suppressing an increase in capacitance between a pixel electrode and a signal line.
In order to achieve the above object, an image sensor of the present invention is characterized by including:
a conversion section for converting an incident electromagnetic wave into electric charge;
pixel electrodes for storing the electric charge generated by the conversion section;
switching elements for controlling reading out of the electric charge from the pixel electrodes;
an interlayer insulating film made of an organic film formed under each pixel electrode;
an electrically conductive film which is electrically connected to the pixel electrodes, and which is extended from the pixel electrode to a layer above each switching element; and
a dielectric layer formed between the switching element and the electrically conductive film.
According to the foregoing structure, an interlayer insulating film is formed between the scanning lines, signal lines, and pixel electrodes in the active matrix substrate. It is therefore possible to form the pixel electrodes over the signal lines. As a result, an improved aperture ratio can be achieved, and in the meantime, by shielding the conversion layer from the electric field generated by the signal lines and the scanning lines, an operation inferior of the conversion layer becomes less likely to occur.
Moreover, the organic film of low dielectric constant can be formed thick with ease, and therefore the electrostatic capacitance between the pixel electrode and the source signal line can be suppressed to be small. As a result, an increase in noise due to an increase in capacitance of the source signal lines can be prevented, and an improved signal to noise (S/N) ratio can be achieved. Furthermore, the active matrix substrate of the image sensor can be manufactured by the manufacturing process of conventional liquid crystal display device without significant modification, and therefore any additional facility is not needed.
Furthermore, the electrically conductive film is extended from the pixel electrode over the switching element. Therefore, even if a normal readout operation of a predetermined cycle is not performed due to any failure, or trouble in signal readout program, and unexpectedly large electric charge is stored in the pixel electrode, the switching element is switched ON at or above a predetermined threshold voltage to release the excessive electric charge, thereby preventing the switching element from being damaged.
Moreover, by forming the dielectric layer between the switching element and the electrically conductive film, such characteristic that the thin film transistor is switched ON at or above a predetermined threshold voltage is determined by the thickness and the dielectric constant of the dielectric layer formed between the electrically conductive film and the switching element, and therefore, the foregoing characteristic can be set independently of the interlayer insulating film. Namely, while maintaining optimal excessive voltage discharge characteristic, the electrostatic capacitance between the pixel electrode and the source signal line can be suppressed, and in the meantime, the signal to noise (S/N) ratio can be improved.
In the foregoing structure, it may be arranged such that in an area above each switching element, the electrically conductive film contacts the dielectric layer without having the interlayer insulating film in between.
In the foregoing structure, it may be arranged such that the switching element is a dual-gate transistor, and the electrically conductive film functions as one of gate electrodes of the dual-gate transistor.
In the foregoing structure, it may be arranged such that each switching element including its channel region is covered with the dielectric layer,
the electrically conductive film is extended from the pixel electrode to an area above the switching element including its channel region, and
in an area above each switching element, the electrically conductive film contacts the dielectric layer without having the interlayer insulating film in between.
In the foregoing structure, it may be arranged such that the electric charge stored in the pixel electrode is a positive charge, and the switching element conducts with an application of a positive bias voltage.
Alternatively, it may be arranged such that the electric charge stored in the pixel electrodes is negative charge, and the switching element conducts with an application of a negative bias voltage.
In the foregoing structure, it may be arranged such that in an area above the switching element, the interlayer insulating film is formed between the dielectric layer and the electrically conductive film.
According to the foregoing structure, in the area above the switching element, which has the roughest surface in the active matrix substrate, formed are not only the dielectric layer but also the interlayer insulating film made of an organic film. With this structure, even such protrusions and recessions of the rough surface, which cannot be absorbed completely by the dielectric layer alone, can be absorbed to a sufficient level. In this structure, even when adopting a conversion layer made of selenium, it is still possible to suppress crystallization due to the protrusions and recessions, and therefore films can be formed under stable conditions.
In the foregoing structure, the interlayer insulating film may be structured such that at least a portion above the switching element is formed thinner than other portion of the interlayer insulating film.
In the foregoing structure, the excessive voltage discharge characteristic is determined by the thickness and the dielectric constant of the interlayer insulating film in the portion between the electrically conductive film extended from the pixel electrode and the switching element. It is therefore possible to set the foregoing characteristic independently of the interlayer insulating film in the portion for use in forming the electrostatic capacitance between the pixel electrode and the source electrode. Namely, with the foregoing structure, an improved S/N ratio can be achieved while maintaining optimal excessive voltage discharge characteristic.
In the foregoing structure, the interlayer insulating film may be structured such that at least a portion corresponding to the channel region of the switching element is formed thinner than other portion of the interlayer insulating film.
In the foregoing structure, a photosensitive organic film may be adopted as the interlayer insulating film.
According to the foregoing structure, in the area above the switching element, which has the roughest surface in the active matrix substrate, formed are not only the dielectric layer but also the interlayer insulating film made of the organic film. With this structure, even such protrusions and recessions of the rough surface, which cannot be absorbed completely by the dielectric layer alone, can be absorbed to a sufficient level. In this structure, even when adopting a conversion layer made of selenium, it is still possible to suppress crystallization due to the protrusions and recessions, and therefore films can be formed under stable conditions.
In order to achieve the foregoing object, another image sensor for converting incident electromagnetic wave into electric charge by each of a plurality of pixel electrodes and outputting image signals by sequentially reading out the electric charge from the pixel electrodes via switching elements, is characterized by including:
an electrically conductive film formed so as to be extended from the pixel electrode to a portion above each switching element; and
an interlayer insulating film made of an organic film, formed below each pixel electrode and the electrically conductive film, the interlayer insulating film being structured such that a portion above the switching element is thinner than other portion of the interlayer insulating film.
In the foregoing structure, the excessive voltage discharge characteristic is determined by the thickness of the dielectric constant of the portion between the electrically conductive film extended from the pixel electrode and the switching element, and therefore, it is possible to set the foregoing characteristic independently of the interlayer insulating film in the portion for use in forming the electrostatic capacitance between the pixel electrode and the source electrode. Namely, with the foregoing structure, an improved S/N ratio can be achieved while maintaining optimal excessive voltage discharge characteristic.
In the foregoing structure, it may be arranged such that the interlayer insulating film is structured such that at least a portion corresponding to the channel region of the switching element is formed thinner than other portion of the interlayer insulating film.
In the foregoing structure, it may be arranged such that the channel region of the switching element contacts the interlayer insulating film.
In the foregoing structure, an inorganic film may be adopted for the dielectric layer.
In the foregoing structure, it may be arranged such that:
a double layer structure of the dielectric film of an inorganic film and the interlayer insulating film of an organic film is formed under the pixel electrode,
in an area above each switching element, the electrically conductive film contacts the dielectric layer without having the interlayer insulating film in between.
In the foregoing structure, it may be arranged so as to further include:
a signal line for transferring charge as collected in each pixel electrode via a switching element,
wherein the pixel electrode is formed over the signal line having the interlayer insulating film in between.
In order to achieve the above object, a method of manufacturing an image sensor of the present invention is characterized by including the steps of:
forming a plurality of switching elements, a plurality of scanning lines and a plurality of signal lines on an insulating substrate;
forming an interlayer insulating film made of a photosensitive organic film in respective portions above the plurality of switching elements, scanning lines and signal lines,
exposing and developing a resulting photosensitive organic film;
forming pixel electrodes on the interlayer insulating film; and
forming conversion means on the pixel electrodes for converting an incident electromagnetic wave into electric charge,
wherein exposure with respect to the photosensitive organic film is varied between at least a portion of an area above each switching element and other portion of the photosensitive organic film.
According to the foregoing structure, the protrusions and recessions resulting from the patterning of the wires in layer below the interlayer insulating film can be suppressed by the interlayer insulating film, and an inferior in characteristics of the conversion means for converting an incident X-ray into electric change in upper layer can be prevented. Moreover, by adopting photosensitive resin, a smooth cross section can be achieved even at the pattern edge of the interlayer insulating film, and therefore it is possible to more surely prevent an inferior characteristic of the conversion means. Furthermore, as the pixel electrodes can be formed over the source electrodes, an area occupied by the pixel electrodes can be increased, and therefore, it is possible to collect the electric charge generated from the conversion means in an efficient manner. In the foregoing structure, even if a normal readout operation of a predetermined cycle is not performed due to any failure, or trouble in signal readout program, and unexpectedly large electric charge is stored in the pixel electrode, the switching element is switched ON at or above a predetermined threshold voltage to release the excessive electric charge, thereby preventing the switching element from being damaged. Moreover, while maintaining optimal excessive voltage discharge characteristics, the electrostatic capacitance between the pixel electrode and the source signal line can be suppressed, and in the meantime, the signal to noise (S/N) ratio can be improved.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.